The invention relates to filters and more particularly, to digital filters which can be used with a radar moving target indicator (MTI).
The purpose of an MTI filter is to ascertain whether or not a target detected by the radar receiver is traveling either toward or away from the radar receiver at a velocity greater than a certain minimum threshold velocity. The MTI filter eliminates from the MTI display targets which are either not moving or have a very low velocity with respect to the radar antenna. Prior art digital filters accomplished this task by performing a series of correlation functions on the incoming radar signal. To perform N correlation functions prior art digital filters required N reference generators, N multipliers and N integrators connected in parallel paths, each of which analyzed a separate frequency spectrum of the incoming signal. If it were desired to broaden the frequency band covered by the filter an additional separate filter would have to be added to the existing filter bank. Expanding the bandwidth of a digital filter in this manner greatly increases the complexity of the circuit.
The present invention completely avoids this complex circuit arrangement by performing N correlation functions in a circuit containing a single reference generator, a single multiplier and a single integrator.
An additional difficulty with prior art filters was the occasional inability to detect a moving target having a Doppler shift frequency lying within the bandpass of the filter bank due to a quadrature phase relationship between the phase angle of the specific reference generator signal and the phase angle of the signal reflected from a radar target. The present invention avoids this problem by providing a second filter channel having a reference generator signal shifted 90.degree. in phase from the other filter channel reference generator signal so that an unknown incoming signal can never be in quadrature with both channels of this digital MTI filter.
Additionally, prior art filters produced a characteristic frequency response wherein the first side lobe was only 13 dB below the peak response. Subsequent side lobes decreased very slowly with frequency. This filter characteristic is unacceptable for most MTI applications. The digital MTI filter of the present invention produces a response curve having substantially reduced side lobes and far steeper filter skirts thereby providing greatly increased selectivity.